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    <title>Minimum TAO</title>
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<strong>Minimum TAO</strong>
<p>

In addition to our work on <A HREF="../../docs/ACE-subsets.html">ACE
subsetting</A>, we have also been reducing the footprint of TAO.  We
are pursuing two complementary strategies to reduce TAO's footprint:

<OL>

<LI> <B><EM>Implicit subsetting</EM></B>, e.g., by reducing
    dependencies in the TAO library so that programs need not link
    unused TAO components. <P>

<LI> <B><EM>Explicit subsetting</EM></B>, e.g., by supporting the <a
href="http://www.omg.org/cgi-bin/doc?orbos/98-08-04.pdf"><em>minimumCORBA
</em></a> specification to <a href =
"http://www.dre.vanderbilt.edu/~schmidt/TAO.html">TAO</a>.  The <em>
minimumCORBA </em> specification removes the following features from
the <a href
="http://www.omg.org/technology/documents/formal/">CORBA</a>
specification. <P>
<ul>
<li>Dynamic Skeleton Interface<br>
<li>Dynamic Invocation Interface<br>
<li>Dynamic Any<br>
<li>Interceptors<br>
<li>Interface Repository<br>
<li>Advanced POA features<br>
<li>CORBA/COM interworking<br>
</ul>
</OL>

The implicit subsetting of TAO requires no explicit application
programmer intervention.  In contrast, to minimize the footprint of
TAO explicitly, you must
<OL>
<LI> <a
href="../../ACE-INSTALL.html">Configure</a> ACE to support only those
components that are required by TAO and <P>
<LI> <a href =
"../../TAO/TAO-INSTALL.html">Configure</a> TAO to only support the
components specified by the <em>minimumCORBA</em> specification.
</OL>

The following two tables show the footprint reduction achievable via
explicit subsetting.  Note that the IDL Compiler column refers to the
code required to collaborate between the IDL compiler and the ORB, and
not to the code for the IDL compiler itself.

<p><hr width=50% align=left> <p>

Note: All measurement are for ACE 5.0 and TAO 1.0 using egcs-2.91.60
on SunOS5.7<P>

The <a
href="../../ACE-INSTALL.html#flags">make
flags</a> options used were: <P>

<code> debug=0 optimize=1 static_libs_only=1</code> <P>

These options translate into:<P>
<UL>
<LI> No debugging
<LI> Optimization is set to -O2
<LI> Static ACE and TAO libraries
<LI> Use of RCS Ids is turned off
</UL>
<p>

To build a TAO static library, if shared libraries are the default,
use <code>make static_libs_only=1</code> (make sure to do this for
ACE, as well).  If you're using recent versions of GNU GCC, you can
use the <A HREF="../../ACE-INSTALL.html#repo">-frepo</A> option, which
typically reduces the footprint by another 25 percent. <P>

<HR>

<h3><a name="Status">ACE+TAO Subsetting Work in Progress</a></h3>

We've been tracking the footprint reduction of ACE+TAO periodically
since April, 2000.  All the statistics are available <A
HREF="http://www.dre.vanderbilt.edu/Stats/footprint.shtml">online</A>.  As the result of
this prior work, we've identified various areas for improvement that
we're now addressing.  For example, the following are the remaining
areas for ACE subsetting:

  <ul>
    <li><EM><B>Log_Msg decoupling</EM></B> --  We have a good start on this, but it needs work
      to finish.  It might be easiest to make an abstract
      base class, then have ACE_Log_Msg derive from it.  That way
      we could remove the exposure of all the #includes in Log_Msg.cpp
      to applications that don't need it.  Another alternative would
      be to disable compilation of Log_Msg.cpp when ACE_NLOGGING is
      enabled.<p>

    <li><EM><B>Higher layer interdependencies</EM></B> -- We haven't
    exhaustively tested all possible combinations of subsets.  It's possible
      that there are interdependencies between some layers that
      we haven't yet identified and removed.<p>

    <li><EM><B>Remove reliance on multiple inheritance</EM></B> -- Multiple
        inheriance is only used in a few
      places in ACE.  This
      isn't really a subsetting problem, but is necessary to
      fully take advantage of optimizations available on C++ compilers for
      embedded systems.<p>
  </ul>

We anticipate that these changes should reduce the default size of ACE
by around 100-200 kbytes.<P>

As the effort to reduce TAO's footprint continues, we are planning
several modifications for TAO that should reduce the footprint for
both the full CORBA and minimum CORBA configurations by around 300-400
Kbytes.  The list below contains an estimate of the impact of each one
of these changes, along with the estimated effort to implement them.
</P>

<P>
  <TABLE CELLPADDING=4 BORDER=4>
    <TR>
      <TD>Component</TD><TD>Impact</TD><TD>Effort</TD>
      <TD>Description</TD>
    </TR>
    <TR>
      <TD>ACE</TD><TD>14 Kb</TD><TD>4 weeks</TD>
        <TD>
  Implement a TAO-specific Reactor.
  ACE's reactor supports a number of features that TAO does not
  require.  Thus, a TAO-specific implementation is an important way to
  reduce the footprint.
        </TD>
    </TR>
    <TR>
      <TD>ACE</TD><TD>20 Kb</TD><TD>4 weeks</TD>
        <TD>
  Implement a TAO-specific Service Configurator.
  TAO uses the ACE Service Configurator to dynamically configure
  its strategies.  In many embedded applications the set of
  strategies are selected at design-time, on those platforms it
  would be appropriate to disable all the features to
  dynamically load components into the ORB.
        </TD>
    </TR>
    <TR>
      <TD>TAO</TD><TD>10-15 Kb</TD><TD>1-2 weeks</TD>
        <TD>
  Eliminate duplicate code due to instantiations of string ->
  pointer maps.  TAO uses several such maps, they could be replaced by a
  generic version, wrapped with a fully inlined (i.e. zero
  footprint) adapter for type-safety.
        </TD>
    </TR>
    <TR>
      <TD>TAO</TD><TD>3-10 Kb</TD><TD>1-2 weeks</TD>
        <TD>
  Make message buffering strategies optional.
  TAO supports policy extensions to control the outgoing oneway and AMI
  request buffers.  Those policies are not used by all
  applications.
        </TD>
    </TR>
    <TR>
      <TD>TAO</TD><TD>10 Kb</TD><TD>2 weeks</TD>
        <TD>
  Make support for multiple ORBs optional.
  TAO can support multiple ORBs in the same process, but most
  applications only require one.
        </TD>
    </TR>
    <TR>
      <TD>TAO</TD><TD>5 Kb</TD><TD>1 week</TD>
        <TD>
  Move the less common transport muxing and reply waiting
  strategies to an optional library.
        </TD>
    </TR>
    <TR>
      <TD>ACE+TAO</TD><TD>30 Kb</TD><TD>8 weeks</TD>
        <TD>
  Decouple ACE (and then TAO) from the
  <CODE>ACE_Thread_Manager</CODE> component.
  This component is only used in the thread-per-connection
  model, if we could decouple it in ACE then TAO could be
  modified to only link this component when that concurrency
  model is enabled.
        </TD>
    </TR>
    <TR>
      <TD>TAO</TD><TD>&gt;50 Kb</TD><TD>6 weeks</TD>
        <TD>
  Move <CODE>&lt;&lt;=</CODE> and <CODE>&gt;&gt;=</CODE>
  operators to separate files.
  Currently TAO includes nearly 500 such operators, moving them
  to separate files (grouped by component?) would eliminate them
  from most applications.
        </TD>
    </TR>
  </TABLE>
</P>

<P>In parallel with the activities described above we are pursuing
other avenues of research to find sources of rarely used or unused
code in ACE+TAO, and to modify the software to eliminate such code.
These activities include the following:</P>

<UL>
  <LI><P>
  Using profiling tools, such as gprof, Quantify and True Coverage
  to find unreachable code, or code only reachable in certain
  applications.
  </P></LI>
  <LI><P>
  The code TAO's IDL compiler generates for <CODE>CORBA::Any</CODE>
  operators is large, so we are evaluating designs that reduce the impact of the <CODE>CORBA::Any</CODE>
  type support.  TAO's IDL compiler already makes that support optional.
  However, for applications that require <CODE>Anys</CODE> it may be useful to separate that code
  in another file to reduce the size of generated stubs and skeletons,
  without losing the opportunity to use more dynamic CORBA invocation
  modes.
  </P></LI>
  <LI><P>
  The Notification Service currently depends on the Trading service to
  implement the Trader Constraint Language.  We are planning to break that
  dependency and factor the TCL parser into a smaller library shared
  by both services.
  </P></LI>
  <LI><P>
  TAO still contains features that are rarely or never used.
  Examples include the interfaces to query the well-known services and
  to dynamically discover the level of security support.  Those
  components should only be linked (dynamically) in applications that require them.
  </P></LI>
  <LI><P>
  The support for interceptors currently generated by the IDL compiler
  can be partially refactored into common ORB code.
  Moreover, we evaluating a new implementation of interceptors
  that can be configured dynamically, thereby eliminating the need for
  compile-time configuration flags.
  </P></LI>
  <LI><P>
  We are planning to provide compile-time flags to eliminate certain mandatory features
  in CORBA that are not used in all applications, such as IOR
  parsers (corbaloc, corbaname, etc.).
  </P></LI>
  <LI><P>
  Finally, we will perfom more code inspections to determine if template code can be
  refactored into base classes and thereby shared by many objects in
  the ACE+TAO implementations.
  </P></LI>
</UL>

Although we do not yet have sufficient insights to know how much
footprint reduction these activities will afford, we anticipate these
enhancements could reduce the default footprint of TAO by another
100-200 Kbytes.

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